The GE Mark VI Gas Turbine Controller was designed so that the generator circuit breaker opens when the gas turbine load is 2.2 MW (100 MW rated power) compared with -1 MW in Mark IV Controller, at which I think safer in terms of overspeed protection when the generator is de-loaded.
The GE representative did not give any reasons on this. Can anyone give me a clue why GE change the setting?
No direct knowledge, but having worked with the protective relaying on many a large machine, here are a couple of thoughts.
The breaker opening on reverse power is not to protect from overspeeding (reverse power into a synchronous machine *can't* overspeed it), but rather protect the prime-mover (i.e. turbine) from damage. In steam turbines, the damage is overheating of the blading, causing thermal growth and rubbing parts. In a gas turbine, I'm not sure what the problem would be, perhaps some gas turbine expert could explain what is damaged by motoring the unit.
Opening at small positive power such as your 2.2 MW would be a problem with traditional, manually loaded machines. When first synchronizing to the grid, the amount of load the machine assumes when the breaker first closes is a function of how much faster than the grid it is running at the point of closure. With really large machines, that are brought on-line with the 'synchroscope going very slowly in the fast direction', having the unit trip back off immediately could be a problem.
But in the newer controls, and I guess in the Mark VI controls you now have, the system 'picks up' load right away when first closing the output breaker, avoiding these 'false trips'.
When taking the unit off line, the exact moment of tripping is not very critical. If there is still a lot of load on the machine when the breaker is tripped, *then* you could have an overspeeding situation. This is why it is standard practice to reduce the load and trip the breaker as the output approaches zero (or let the reverse power circuit trip the output breaker).
Running power down 0, the machine will speed up some. Perhaps the controls are designed to keep the speed up so it can be brought back on line quickly if needed?
daestrom
The two main reasons for reducing load before opening the breaker is to reduce thermal stress on the set by uniformly and slowly cooling the turbine and secondly to reduce the affect of instantly loading grid equipment and the subsequent affects on other connected consumers.
On a 100MW system or any other generating system changing from 1% to 2.2% is really irrelevant and has probably been changed without a conscious effort or discussion.
It all depends on your perspective for reasons when a breaker is opened. So from GE's perspective I'm guessing it may be that a default setting has been left as that.
Newsey
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